One such safety system is disclosed, for example, in German patent publication 3,502,100 where it is utilized in a hydraulic brake system for a motor vehicle. In normal operation, the hydraulic brake system is supplied from the electrical energy source in the form of a battery. The instantaneous operating voltage is monitored by a voltage-monitoring unit. An instantaneous operating voltage, which drops below a limit value because of a malfunction of the battery, is detected by the voltage-monitoring unit whereupon the monitoring unit addresses a switchover element assigned thereto. The switchover element connects the electrical consumers of the hydraulic brake system to the electrical energy store. The battery is charged by a generator and, as soon as a rated voltage is applied to the battery, the switchover element is again addressed by the voltage-monitoring unit and the switchover element again switches back to the normal operation of the hydraulic brake system. If the instantaneous operating voltage drops below a limit value at the electrical energy store as a consequence of the discharge, this is likewise detected by the voltage-monitoring unit which then addresses a charge switch unit which then connects the second electrical energy store to the generator so that this second store too can be charged.
The foregoing shows that the hydraulic brake system disclosed in German patent 3,502,100 can be characterized as safe because the electrical consumers of the hydraulic brake system are supplied by a second electrical energy store when there is a malfunction of the battery. The instantaneous operating voltage of the battery must, however, be continuously detected by the voltage-monitoring unit and, in the case of a fault, this monitoring unit must drive the switchover element. A switchover from the battery to the second energy store is no longer possible if the voltage-monitoring unit malfunctions or if there is an incorrect measurement of the instantaneous operating voltage or if there is a malfunction of the control line from the voltage-monitoring unit to the switchover element. In these cases, the reliability of the hydraulic brake system is affected. Furthermore, the safety system disclosed in German patent publication 3,502,100 has a complicated configuration.